We are examining chemoreceptor function, respiratory center output and cardiac rate reactivity in two groups of infants believed at high risk of sudden infant death syndrome (SIDS). Group one are infants born to mothers addicted to methadone; group two are infants who have had unexplained apnea or cyanotic spells during early infancy. Chemoreceptor function will be evaluated by determining the ventilatory response to steady-state carbon dioxide inhalation. Respiratory center output will be assessed by measuring the pressure response at the nose to brief (0.1 sec.) airway occlusion, while breathing room air and carbon dioxide. Cardiac rate reactivity will be assessed by determining acceleration, deceleration and beat-to-beat variability responses to acoustic and airstream stimuli. During all cardiopulmonary evaluations, sleep state will be monitored by electroencephalogram (EEG), electrooculogram (EOG) and electromyogram (EMG). The cardiopulmonary responses of methadone exposed infants and infants subject to apnea spells will be compared to normals taking into account the level of inspired oxygen, gestational and chronologic age and sleep status. Identification of infants with dysfunction of chemoreceptor, respiratory center or cardiac rate reactivity responses in these two groups at high risk for SIDS may allow definition of abnormalities that predispose seemingly normal infants to SIDS.